1. Technical Field:
The present invention relates in general to power conversion and, in particular, to DC/DC power converters having improved topologies that provide high efficiency with low output impedance. More particularly, the present invention relates to a high-efficiency power converter and a method of operation thereof.
2. Description of the Related Art
Regulated DC power supplies are typically needed for most analog and digital electronic systems. Two major categories of regulated DC power supplies are linear power supplies and switching power supplies. Generally, in linear power supplies, a transistor (operating in its active region) is connected in series with an input voltage source and the voltage drop across the transistor is automatically adjusted to maintain an output voltage at a desired level.
In switching power supplies, transformation of DC voltage from one level to another is accomplished typically by means of DC/DC converter circuits, such as step-down (buck) or step-up (boost) converter circuits. Solid-state devices, such as transistors, are operated as switches (either completely ON or completely OFF) within these switching converters. Since the power devices are not required to operate in their active region, this mode of operation results in lower power dissipation. Furthermore, increasing switching speeds, higher voltage and current ratings of these power devices are some of the advantages that have increased the popularity of switching power supplies. It is not uncommon for switching power supplies to have a switching frequencies of 100 kHz or higher. The high switching frequencies permit the components utilized in construction of the power converter, such as capacitors, inductors and transformers, to be physically small. The reduction in the overall volume (size) of the converter that results is desirable to the users of such supplies. Another important attribute of a power supply is its thermal efficiency. The higher the efficiency, the less heat that is dissipated within the supply, and the less design effort, volume, weight, and cost that must be devoted to remove this heat.
Distributed power systems are increasingly being utilized in modern systems, for example, due to different power levels requirements in a power consuming system. Furthermore, with the increasing popularity and utilization of complementary metal oxide semiconductor (CMOS) technologies in present and future projected electronic systems, highly dynamic loading conditions are a common problem encountered by power systems. Highly dynamic loads present unique problems to a supply power system. This is especially true with load systems, such as computer systems, utilizing complementary metal oxide semiconductor (CMOS) technologies. In these systems, it is not uncommon for portions of the load system to switch from ten percent to full load in a few microseconds. This load behavior makes it difficult to maintain the power system bus voltages in regulation. One approach to resolve the widely varying output load swings is to utilize large decoupling capacitors at the load. W This approach provides some load regulation relief, however at low bus voltages, the amount of decoupling required becomes unmanageable. To illustrate, at a given power level, four times as much decoupling is required to maintain a 2.5V bus within a specified percentage tolerance as for a 5V bus. Since electrolytic capacitor impedance is primarily dominated by equivalent series resistance and inductance at high frequencies, and these parameters are determined mostly by case size (independent of the voltage rating of the capacitor), four times as much space must be allocated to decoupling at the lower voltage level as at the higher voltage level.
Accordingly, what is needed in the art is an improve power converter topology that mitigates the limitations in the prior art. More particularly, the highly dynamic loading conditions necessitates a low impedance power distribution network driven by low impedance power supplies.
To address the above discussed deficiencies in the prior art, and in accordance with the invention as embodied and broadly described herein, a high efficiency power converter is disclosed. The high efficiency power converter includes a preregulator circuit coupled to a low impedance output stage. The preregulator circuit provides a regulated intermediate DC voltage and includes a regulator switching device that is coupled to a source of DC power and a regulator inductor. The low impedance output stage includes first and second power transformers, where each of the power transformers has first and second primary windings of opposite polarity and a secondary winding. The low impedance output stage also includes first and second switches that alternatively couple the first primary windings of the first and second power transformers, respectively, to the regulated intermediate DC power. The first and second switches, in an advantageous embodiment, are Field Effect Transistors (FETs) and are driven in a complementary manner with a duty cycle of about 47% to 49%. The low impedance output stage further includes first and second coupling capacitors that couple the second primary windings of the first and second power transformers to the second and first switches, respectively, and are utilized to reset the first and second power transformers with minimum losses. In a related embodiment, the low impedance output stage also includes third and fourth switches that are coupled to the secondary windings of the second and first power transformers, respectively, and are driven in a substantially identical manner as the second and first switches, respectively.
The present invention discloses a novel high efficiency low output impedance power converter having a preregulator circuit that provides a regulated intermediate voltage to a low impedance output stage. A coupling inductor coupled to an inductor in the preregulator circuit is utilized to provide a means of transferring the remaining energy present in the inductor to an output device, such as an output capacitor, when the switches in the output stage are turned off. This, in turn, will reduce the output ripple current and improve the transient response of the power converter. The present invention also recognizes that the utilization of the coupling capacitors in the output stage, in contrast to prior art power converters employing devices, such as diodes, improves the overall efficiency of the power converter due to their xe2x80x9closslessxe2x80x9d nature. The improved efficiency is due to the substantial elimination of energy losses, i.e., thermal energy, that are a consequence when devices such as diodes are utilized. Additionally, unlike diodes, the current can flow from both directions into the coupling capacitors during their charging and discharging cycles, resulting in zero-voltage-switching (ZVS) during the turn ON of the first and second switches. Furthermore, since the coupling capacitors can charge and discharge in both directions, the currents through the first and second switches are the combination of a primary winding current and the current through a coupling capacitor. Consequently, since the coupling capacitors are also contributing to the current flow through the switches, the efficiency of the power converter is improved.
The foregoing description has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject matter of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.